Tighten up some error and state checks in the crypto code.
Tighten up some error and state checks in the crypto code.

* subversion/libsvn_subr/crypto.c
  (RANDOM_PREFIX_LEN): New #define, to replace ...
  (PREFIX_LEN): ... this one.
  (svn_crypto__context_create): Delay creation of the context
    structure until other bits succeed.  (Also, collapse and amend
    some comments.)
  (svn_crypto__encrypt_password): Verify that the key returned from
    apr_crypto_passphrase() is not NULL, and that the required IV
    length is non-zero.
  (svn_crypto__decrypt_password): Verify that the key returned from
    apr_crypto_passphrase() is not NULL, that the required IV length
    is non-zero, and that the input IV has the correct length.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion